1) No, you can't overclock it in BIOS. Dell uses a proprietary motherboard and BIOS so overclocking options have been hidden from the user.
2) Even if somehow you manage to find overclock settings in there, I strongly advise not touching them. There are ways to do it without going through the BIOS too, but I leave that as a googling exercise for the reader. If I am not mistaken, the Inspiron 560 is pretty much a bottom-of-the-line budget option, which means it comes with a minimal CPU cooler, PSU and other parts; nothing like the kind of case-build engineering that goes into an XPS.
That's not to say it can't be done, but any sane person will tell you that overclocking a bottom-budget prebuilt is a bad idea unless you're just doing it for fun and have another main machine. A successful overclock also involves picking hardware that won't curl up in a corner and cry when you attempt a good overclock, and somehow I doubt you're going to get hardware like that on a shoestring budget.
